The Bank of England‘s confidence in the ability of its unorthodox bond-buying program to generate further growth seems to be wavering. Some of the recent comments from members of its policy-setting committee suggest caution about extending quantitative easing.

In part that’s because they’ve taken heart from early evidence that the BoE’s Funding for Lending scheme, in which it subsidizes bank lending, is managing to push credit into the system. And, no doubt, there’s a desire to keep policy as stable and uncontentious as possible ahead of the change in the Bank’s leadership this summer.

But there also seems to be a rethink of the ultimate costs of the BoE’s hugely expanded balance sheet relative to the seemingly shrinking incremental benefits of more bond buying, not least because the policy-committee members are unsure about what the economy’s sustainable trend rate of growth might be.

Is it really a coincidence that commodity prices started soaring again as expectations kicked in of yet more quantitative easing by the U.S. Federal Reserve and as other central banks opened their liquidity taps?

Yes, there’s a drought in the U.S., and Middle East turmoil. Corn and soybean prices have rocketed to new highs, while wheat prices are up 50% since the start of last month as the U.S. suffers from a heatwave and the worst drought in half a century. And concerns about the Syrian civil war and ongoing tensions between the West and Iran have squeezed oil prices sharply over the same period.

But there’s always a drought somewhere, and Middle East turmoil is the norm. Looking back over the past five years, commodity prices have tended to spike hard in the wake of central bank easing.

In the days of innocence, when no one knew what “subprime” meant and the idea of Lehman Brothers going critical would have been hilarious, slower inflation usually meant the currency of the country in question took a dive.

Before “Quantitative Easing,” when conventional monetary policy seemed to work so well as to be magical, weaker inflation simply meant the central bank would nudge interest rates down to attempt to kickstart a flagging economy. All very well, but lower rates meant offshore investors would have less incentive for holding a nation’s bonds or depositing money in its banks. Down went the currency. Simple.

In our crisis-ridden world, however, news that consumer price inflation in the United Kingdom was slower than it’s been for two and a half years might actually be good for the pound, according to Kit Juckes, head of foreign-exchange research at Societe Generale.

With conventional policy at its limits, interest rates aren’t likely to go anywhere. They’ve been stuck at a record low of 0.5% since March 2009.

But, “the U.K. inflation data were friendly for sterling bulls, although that may not have seemed the case on first glance,” he wrote.

What happens to a currency when its monetary authority presses the quantitative-easing button?

We all used to think we knew. Surely money-printing was a significant currency negative, for very good reason. It is inherently inflationary, and inflation erodes currency value. And, more basically, there’s simply more of your currency around after QE; supply has gone up. Unless demand does too, even those who’ve just started their economics courses know what happens next.

Well, that was the theory. Of course, back in the pre-crisis paradise, it could remain theoretical as there was no chance at all that Western central banks would ever need to indulge. Conventional monetary policy was the silver bullet. Nothing else was needed.

Well, we know better now but, sure enough, in the first rounds of QE from the Bank of England, the old theory was born out.

Back in 2008, when the BoE expanded its balance sheet from around £100 billion ($1.58 billion) to nearly £300 billion, sterling duly fell. The central bank’s trade-weighted sterling index dropped from above 90 to the mid 70s.

However, things seem to have changed. The theory no longer holds and QE need not bash your currency. After all, sterling has remained broadly stable since the BoE expanded its asset-purchase program in October 2011 and February 2012.

Now, it seems, with many Western governments stymied in their spending plans by the pressing need for austerity, an activist central bank can be supportive for a currency, no matter what form that action takes.

Two of the world’s big central banks, the Banks of Japan and England, have indulged in it this month alone. All the same, there is a growing suspicion that its best years are behind it.

Indeed, it was BOE governor Mervyn King who sowed a seed of doubt as to its future just this week. His outfit may have just increased its own QE program by £50 billion ($79 billion) in February, to £325 billion. But, in his open letter to the Chancellor of the Exchequer that he’s required to pen in the depressingly usual event of an inflation overshoot, Mr. King reminded Her Majesty’s Treasury that there is a limit to what monetary policy can achieve when real economic adjustments are also required to move an economy on.

It seems a long time ago now but, back in the innocent days of conventional monetary policy, consumer price data releases in the U.K. were blue riband events for sterling investors.

It was only natural that they should be. After all, the annual increase was targeted by the Bank of England and so had a direct bearing on policy settings. CPI breakdowns were anxiously parsed for interest rate cues.

However, we live in more interesting times.

To be sure there remains an official CPI target of 2%; but it’s been missed for 24 months straight, even if January’s 3.5% rise was the lowest since November 2010.

Quantitative easing is meant to support asset prices and, via the wealth effect, to boost consumption and economic growth.

But does it?

Japan’s forays into QE over the past two decades, and the U.S.’s since the financial crisis in 2008, suggest that QE has only a temporary impact on share prices at least, according to Sushil Wadhwani, a former member of the Bank of England’s Monetary Policy Committee and a hedge fund manager, in a commentary for the Financial Times.

In other words, the Bank of England shouldn’t hold out too many hopes for the £75 billion in government bond purchases the MPC launched this month, adding to the £200 billion in asset purchases it has already done.

At best, it seems, QE offers only a temporary boost that fades once the actual purchasing program ends.

But that’s only half the story.

QE is bound not to work. Unless, of course, it ends up working too well.

Talk has it that QE is now a bad word in the corridors of Number 10 Downing Street.

And there is one very good reason why: inflation.

Upward pressure on consumer prices has remained stubbornly high at the same time that the U.K. economy continues to falter.

In other words, the recovery that might have made inflation more bearable hasn’t materialized and the government is now left with the political hot potato of a population watching its standard of living fall as inflation erodes its spending power even more.

Further evidence of the squeeze was apparent on Friday, when John Lewis, a key barometer of middle-class shopping, reported a 1.7% fall in sales excluding VAT last week compared with a year ago. Other data showed that consumer confidence had ticked up a little but it is still at one of the lowest levels in the past 37 years.

In a world of compressed yields, investors taking on enormous risks in order to make small gains has been likened to picking up pennies in front of a steamroller.

That’s what helped to bring on the financial crisis. Is that what central banks are doing now with quantitative easing? Or, to make the metaphor a little more precise, are they standing in front of steamrollers doling pennies out? In a world of zero interest rates, central banks are constrained in what they can do to loosen monetary policy, so they buy government bonds and thus pump liquidity into the financial system, a mechanism known as quantitative easing.

The problem is that there’s a great deal of uncertainty about how QE works–it may do so via the wealth effect by forcing asset prices higher or by encouraging long-run investment by driving down long-run interest rates or by boosting inflation expectations or increasing actual inflation–or how effective it is.

Mr. Cable’s remarks would seem to suggest he favors a resumption of QE to tackle the “serious threat” to the U.K. economy posed by weak demand.

He’s said something similar in an interview in July, while recent comments by George Osborne, the U.K. Chancellor of the Exchequer, about the scope for “monetary activism” would suggest he too isn’t averse to more money printing.

Of course, the problem for Mr. Cable and his political colleagues is that QE isn’t in the government’s toolbox. It’s in the Bank of England’s, and it has yet to oblige.